The conventional technique for commercial production of sodium bicarbonate dissolves soda ash in a spent reaction liquor from a prior reaction, consisting of water containing small quantities of dissolved soda ash and sodium bicarbonate. The solution is then carbonated to precipitate crystals of sodium bicarbonate. The sodium bicarbonate crystals are separated from the liquor and dried to yield highly purified, high density crystals. Disadvantages of the conventional method are that the procedure requires several steps, and necessitates using of separation equipment, drying of the product, and handling of large volumes of liquids.
Sodium bicarbonate may also be produced by various dry carbonation techniques. In U.S. Pat. Nos. 276,990 and 574,089, a sodium bicarbonate product is formed by placing hydrated soda ash in a revolving cylinder and then introducing carbon dioxide into the cylinder. Both patents teach reaction times of approximately five to six hours.
U.S. Pat. No. 3,647,365 teaches a process in which hollow sodium bicarbonate beads of low density are prepared in a multistage reactor from hydrated soda ash, small amounts of water and carbon dioxide. This process requires several steps and must proceed slowly, with carbonation times exceeding one hour and drying times taking up to eight hours. Further, the soda ash must first be hydrated in a separate step and the reaction must occur at a temperature above 95.7° F. to achieve a commercially acceptable reaction rate.
More recently, U.S. Pat. No. 4,459,272, discloses a process for the preparation of sodium bicarbonate by the reaction of a solid, particulate sodium carbonate-containing material with liquid water in a carbon dioxide-rich atmosphere. The particulate mass is mixed in an internally agitated or externally rotated or vibrated reactor with the water and carbon dioxide. The reaction is carried out at temperatures from 125° F. to 240° F. under atmospheres containing from 20% to 90% carbon dioxide by volume. The process is carried out under reduced water vapor partial pressures to promote evaporation of water from the surfaces of the reacting carbonate particles, and to maintain high carbon dioxide partial pressures in the reactor atmosphere. Products formed by the process have apparent bulk densities as high as 50-60 lbs/ft3.
Each of the previously described dry carbonation techniques is subject to particular disadvantages. In each process, the carbon dioxide concentration must be high and the reaction temperature must also be high, or the reaction rate is prohibitively low. None of these methods can produce sodium bicarbonate at low temperatures and low carbon dioxide concentrations, at commercially acceptable reaction rates.
Sodium bicarbonate has also been produced, as well as utilized, in dry sorbent injection processes for removing sulfur dioxide emissions from the combustion gases of fossil fuel fired burners. Such techniques have commanded considerable attention recently, particularly since they present the lowest “first cost” alternative for removing potentially dangerous sulfur dioxide from flue gases. Sodium bicarbonate has been demonstrated to be a very effective sorbent for dry sorbent injection processes. However, the cost of pharmaceutical grade sodium bicarbonate, as currently produced, is a major drawback to its use for such purpose.
U.S. Pat. Nos. 3,846,535 and 4,385,039 disclose methods for regenerating sodium bicarbonate from sulfate-containing solid waste formed by dry sorbent injection with sodium bicarbonate. According to U.S. Pat. No. 3,846,535, a regeneration step is accomplished by forming an aqueous solution of the sodium sulfate containing waste, and treating such solution with ammonium bicarbonate to precipitate sodium bicarbonate. The sodium bicarbonate is then separated, dried and recycled for further use. According to U.S. Pat. No. 4,385,039, a regeneration step involves dissolving the solid desulfurization reaction product in a basic liquor, which contains borate ions and/or ammonia. Carbonation of this liquor results in a sodium bicarbonate precipitate. Each of these disclosed processes suffer from the use of complicated and capital-intensive equipment and solution operations.
An improved process for the production of sodium bicarbonate for use in dry flue gas desulphurization processes that does not require the multiple operations and systems required by prior art processes is needed. Further, a process to produce bicarbonate sorbent directly employed in the desulfurization of flue gases, that is efficient and economical, is needed.